One of the links provded by Michael Karas provided the signal conditioning parameters I was looking for. So I ended up feeding the piezo signal into a voltage divider (22k and 4.7k) to reduce the signal level to a good range, then through a rectifying diode into a R/C circuit that captures the sample for several milliseconds (1 Mohm and 2200 pF). I also have a reverse biased 5.1v zener across that to clamp any potentially damaging peaks.

The signal is fed into a ADC0809 chip. I will have a total of 12 piezos so I need two ADC0809s. This is actually an advantage over a single larger device because I can get two simulataneous ADC conversions going, and I will wire-OR the EOC pins to the 8751 so that I can then quickly read both chips once both have finished sampling and get a new sample going before I process the inputs. By going through 6 addresses I can keep the total time between two samples of any given piezo to less then 1 mSec, which is good.

A prior search on this forum gave me the information needed to get the MIDI baud rate (31250) with the 8751:

http://www.8052.com/forum/read/117992

I have been testing it (breadboarded) with two piezos connected and I'm able to play velocity sensitive drums already! The next step is to add a one line LCD display in 4 bit mode and some buttons, for things like setting the MIDI note number to send for each drum, and triggering threshold and dynamic range. I've already tested this by hardcoding values for the time being but I'll be adding a slope and offset (the ole Y=mX +b thing) parameters for each drum.

As for the physical drums, I'll be using a used "Band Hero" drum set that I bought for $20, and adding more pads from another similar game controller. They already have the piezos built into them.

This has been a lot of fun so far.